This work considers in detail the EU law and case law affecting various types of "horizontal" agreements - those between undertakings operating at the same level of the manufacturing, supply or retail chain. It derives from a section in the looseleaf Law of the EU (Vaughan & Robertson, eds), and is made available here for the benefit of those who don't subscribe to the looseleaf. Part I analyses so-called 'object'-type agreements which have always been a primary focus of anti-trust enforcement agencies and are likely to infringe Article 81 EC. Part II examines so-called 'co-operation' agreements which either do not infringe Article 81(1) EC at all, or, if they do create an appreciable restriction on competition within the meaning of that provision, may nevertheless benefit from the exception set out in Article 81(3). The way in which the European Commission treats these types of agreements has changed recently and reflects a much less form-based, and more fact-specific, economics-based approach. The texts of relevant Commission Guidelines on the application of the relevant EC Treaty articles, in particular on specialisation and research and development agreements are included in appendix.